Sewage gas contains hydrogen sulfide and other organic sulfides which cause it to be malodorous. Also, the majority of the chemical compounds which cause the odor in sewage gas also cause it to be toxic and corrosive. Numerous sulfur-containing substances have been identified as causing the odor in sewage gas. Examples of these compounds are allyl mercaptan, amyl mercaptan, benzyl mercaptan, crotyl mercaptan, dimethyl sulfide, ethyl mercaptan, hydrogen sulfide, methyl mercaptan, propyl mercaptan, sulfur dioxide, tert-butyl mercaptan, thiocresol and thiophenol, to name a few.
However, hydrogen sulfide is generally one of the main components of sewage gas, being usually contained in relatively high concentrations therein. Accordingly, the degree to which hydrogen sulfide is present in a sample of sewage gas is used as a measure of the odor intensity and corrosiveness of that particular sample.
Not only will hydrogen sulfide cause an intense odor in sewage gas, this compound can have numerous physiological effects and can be quite hazardous. For example, the odor associated with hydrogen sulfide ("rotten eggs") can be detected when the concentration of the hydrogen sulfide is as low as about 0.1 parts per million of sewage gas. As the concentration of the hydrogen sulfide increases, various physical effects are seen, such as headache, nausea, throat and eye irritation, etc. Once the hydrogen sulfide level reaches a concentration of about 500 parts per million of sewage gas or more, serious life threatening effects will result, such as pulmonary edema, nervous system stimulation and apnea. If the hydrogen sulfide level were to reach a concentration of between about 1,000 to 2,000 parts per million of sewage gas, respiratory collapse and paralysis resulting in death may result.
Traditional sanitary sewer design practice has not fully acknowledged the importance of eliminating corrosion and controlling the odor caused by sulfides. This is evidenced by the widespread occurrence of these problems in conventional sewage treatment systems. In conventional systems, odor problems are managed by ventilating sewer systems so that the sewage gas becomes diluted with air. Although this practice may reduce gas concentrations to less than toxic levels and may be useful for controlling corrosion, large volumes of malodorous gas are produced.
In order to address this odor problem, such air-diluted sewage gas is often further chemically treated. For example, offending odors can be made less objectionable through the use of odor-masking and counter-active agents such as vanillin and juniper oil. An example of this method is disclosed in, e.g., EPA Design Manual, "Odor and Corrosion Control in Sanitary Sewage Systems and Treatment Plants", EPA/625/1-85/018, pp 71-93 (1985). However, this approach merely involves replacing an objectionable odor with a more pleasant one. Accordingly, this method is generally the least preferred of the available techniques for reducing sewage gas odor.
Also, strong oxidizing agents, such as chlorine, hydrogen peroxide and strong alkalis, such as sodium hydroxide and lime, have been used to react with the offending substances present in sewage gas. An example of these methods is set forth in "Odor and Corrosion Control in Sanitary Sewage Systems and Treatment Plants", Bowker et al, Noyes Data Corporation, pp 52-60 and 71-78 (1989). However, these approaches have generally not proven to be commercially and/or economically successful since, although removal rates may appear high, the concentration of malodorous components in the treated sewage gas remains above threshold levels for odor detection.
Since such sulfide-containing sewage gas or sludge gas (i.e., gas resulting from waste water or waste water constituents which have undergone anaerobic decomposition) is present in nearly all conventional sewage treatment systems, it can be seen that there is a need for an effective and efficient method for reducing the levels of hydrogen sulfide and other organic sulfides in waste water treatment systems. Such a method is needed, not only to remove the offensive odor and corrosivity associated with the sewage gas, but to reduce the possible occurrence of the adverse physiological effects discussed above.